Fast Photoluminescence and Energy Transfer in Silicon Quantum Dots

Si nanocrystals (SiNC) can be relatively efficient light emitters despite their indirect bandgap. Their attributes (e. g. light-emission efficiency) depend heavily on surface passivation. Various schemes were exploited over the years, however, the functionalization with diphenylanthracene (DPA) is quite new in the SiNC field. To investigate the role of DPA we compared the samples with DPA covalently attached to SiNCs (SiDPA), samples passivated with dodecane (SiDDE) and also the mix of SiDDE with various concentrations of DPA. This enabled us to properly characterize the energy transfer between the DPA molecules and SiNCs.<br/>Apart from the classical methods of optical spectroscopy like absorption and time-resolved photoluminescence (PL) measurements, we also used the seldom utilized two-photon excitation to investigate the energy transitions in our samples in detail. This excitation theme interacts with the samples differently compared to the common one-photon excitation due to the different absorption coefficients of two-photon absorption. The samples exhibit typical two-part PL consisting of the fast (ns) blue band and slow (us) red band. By using the two-photon excitation, we observed the enhanced red PL compared to the blue PL and also completely different dynamics even in the case where the same energy transition was excited, i.e. the two-photon excitation scheme used the twice as long wavelength compared to the one-photon excitation. These new effects were observed even in the “classical” SiDDE sample showing that there are still issues to be resolved, particularly in the blue band area. Our usage of a femtosecond laser and optical parametric amplifier (output pulses with wavelengths between 315 - 2000 nm and time-length 150 fs) with streak camera with ps time-resolution for detection enabled us to analyze the individual decay channels and determine the “transition energy” connected to the switching PL dynamics among other effects.